In typical fermentation methods, a percolate flows through a substrate in a fermentation digester and is collected primarily at the bottom of the digester, returned to a percolate container and from there can be fed to the digester again. This creates methane-containing biogas in the digester and also in the percolate container, said biogas being discharged from the digester and/or the percolate container and fed to a biogas treatment system to increase the methane concentration. With biogas treatment, the biogas to be obtained is separated from other undesired components and is supplied for further use. Among other things, the treatment produces a significant amount of CO2 gas.
To supply the biogas at the end of the process from the digester to the biogas treatment system, the biogas-containing volume above the substrate must be purged after the fermentation process using purge gas in order to eliminate the biogas from the volume. EP 1 997 875 A1 discloses the use of CO2-containing gas from the treatment system for this purpose.
The fermentation process also generates a considerable amount of biogas, which is, however, “trapped” in the substrate itself. This is released during substance handling or reprocessing of the digistate. Thus, the released biogas portions, in particular methane, are not recovered and enter uncontrolled into the atmosphere.